The present invention relates to a process unit to be loaded into an image forming apparatus. The process unit is a unit, for processing image formation, which makes the maintenance of the image forming apparatus simple and easy because the process unit.
An image forming apparatus such as an electrophotographic copying apparatus or printer comprises the following units for processing image formation: a photosensitive drum consisting of a cylindrical base body, having an axis for rotating the cylindrical base body and a photosensitive layer mounted around the outer wall surface of the cylindrical base body; a charging unit for charging the photosensitive layer; a light irradiating unit for forming an electrostatic latent image on the photosensitive layer by irradiating a scanning light in an axial direction of the cylindrical base body in accordance with an electrical image signal; a developing unit for developing the electrostatic latent image on the photosensitive layer so as to form a toner image on the surface of the photosensitive layer in accordance with the latent image; a transfer unit for transcribing the toner image on an appropriate paper; an erasing unit for extinguishing the electrostatic latent image; and a cleaning unit for cleaning the toner left on the surface of the photosensitive layer.
As time passes, these units relating to the image formation are worn and become dirty, so that their characteristics and performance are changed and deteriorated respectively. Therefore, these units have to be cleaned or interchanged by an expert. Above all, the photosensitive unit and the charging unit are required to be interchanged. For solving this problems, some of these units, for example, the photosensitive drum and the charging unit are previously integrated into a unit called a "process unit", and the process unit is interchanged. Sometimes, the developing unit and the cleaning unit are also integrated into the process unit. As seen form the above, the light irradiating unit, the transfer unit and the erasing unit are not required to be integrated because these units are not required to be interchanged so often, so that these units are connected to a main body of the image forming apparatus and only apertures for these units are provided in the process unit. Because of providing this type of process unit, the problem of cleaning and interchanging the units and parts by the expert is eliminated, so that the maintenance of the image forming apparatus can to be performed by interchanging the process unit without calling the maintenance expert.
Usually, in the process unit of the prior art, a corona discharging type charger, which will be simply called a "corona charger" hereinafter, has been used in the charging unit. However, there has been a problem that the corona charger requires a high voltage, such as several kilovolts (kV) for corona discharging and ozone is generated during the corona discharging. Because of the necessity of the high voltage, a high voltage power supply must be provided on the main body of the image forming apparatus, resulting in increasing the size, weight and the cost of the image forming apparatus. And, because of the ozone generation, the photosensitive drum tends to be damaged, resulting in shortening the life of the photosensitive drum, therefore, shortening the life of the process unit. In the process unit using the corona charger, ozone protection means, such as an ozone filter and an ozone exhauster, for protecting the photosensitive drum from the ozone damage has been provided to the process unit. This ozone protection means is effective to prolong the life of the process unit; that is, by employing the ozone protection means, an average of 5,000 sheets can be copied by one process unit. However, this ozone protection means makes the process unit complicated, the size and weight of the image forming apparatus large and heavy respectively, and the cost of the process unit high. Furthermore, this means pollutes the air around the image forming apparatus by the exhausted ozone. The present invention intends to solve the above problems of the process unit.
The prior art process units have been disclosed in the following patents and publication:
U.S. Pat. No. 4,538,896 (1985); PA0 U.S. Pat. No. 4,575,221 (1986); and PA0 Japanese Unexamined Patent Publication (Kokai) No. 59-26745.
All of the above prior art process units include the corona charger. In this statement, the prior art process unit will be explained below with reference with U.S. Pat. No. 4,575,221.
FIG. 1 is a diagram for illustrating a prior art process unit 100 disclosed in U.S. Pat. No. 4,575,221. The process unit 100 is provided in the image forming apparatus, and includes a photosensitive drum 101, a corona charger 102, a developing unit 103 and a toner cleaner 104; wherein, the corona charger 102 is connected to a high voltage power supply 105 of the image forming apparatus, provided exteriorly to the process unit 100. The process unit 100 is effective for facilitating the operation and the maintenance of the image forming apparatus; however, as mentioned before, the corona charger 102 produces ozone, resulting in damaging the photosensitive drum 101. The photosensitive drum 101 is composed of a cylindrical base body 1012 and a photosensitive layer 1011 provided around the outer wall surface of the cylindrical base body 1012. The photosensitive layer 1011 is made of inorganic sensitive material such as zinc oxide or a selenium compound. However, recently organic photosensitive material composed of organic compound tends to be applied to the photosensitive layer 1011 instead of the inorganic photosensitive material. This is because the inorganic photosensitive material is injurious to the human body, so that the process unit using the inorganic photosensitive material as the photosensitive layer 1011 cannot be disposed freely. In contrast, the organic photosensitive material is not injurious, so that the process unit using the organic photosensitive material for the photosensitive layer 1011 can be disposed any place. Moreover, the cost of the organic photosensitive material is lower than that of the inorganic one. However, there is a problem in that the organic photosensitive material is easily damaged by ozone. Consequently, the prior art process unit using the corona charger has the following problems:
1) because of ozone generation, the photosensitive layer is easily damaged;
2) because of ozone generation, the life of the photosensitive layer cannot be made very long even though the generated ozone is exhausted from the process unit, resulting in decreasing the life of the process unit;
3) because of ozone generation, the air around the image forming apparatus is polluted when the generated ozone is exhausted from the process unit;
4) because of ozone generation, the economical, non-injurious and easily disposable organic photosensitive material is hard to be applied to the photosensitive layer;
5) because a high voltage power supply must be provided in to the image forming apparatus, the size, weight and cost of the image forming apparatus cannot be reduced; and
6) because the prior art process unit uses the corona charger, it is essentially hard to charge the photosensitive layer uniformly because a clogging problem occurs in the corona discharging when time passes, producing an irregular charge on the photosensitive layer.